1. Field of the Invention
The present invention relates to a driving circuit for a display apparatus which can display an image with plural gradations by the application of voltages in accordance with digital data.
2. Description of the Related Art
FIG. 8 shows part of a conventional driving circuit for an active matrix type liquid crystal display apparatus utilizing a TFT (thin film transistor). In the following description, the display apparatus is presumed to display an image with four gradations by using two bits of data for simplification. FIG. 8 shows only a portion of the driving circuit contributing to the supply of an output O.sub.n to a data line (the nth data line).
Data D.sub.0 has one bit and Data D.sub.1 has one bit, which are serially sent to the driving circuit, are latched in a sampling circuit 1 by a sampling signal T.sub.SMPn for each data line. The data latched in the sampling circuit 1 are then latched at one time in a holding circuit 2 by a holding signal LP. A decoder 3 decodes the data latched in the holding circuit 2, thereby turning on one of four analog switches 4. As a result, one of four gradation voltages V.sub.0 to V.sub.3 corresponding to the data is supplied to the data line as an output O.sub.n.
In a liquid crystal display, it is necessary to avoid the application of DC components for preventing the degradation of the liquid crystal as a display medium. Therefore, the driving circuit adopts AC driving. In AC driving, applied voltages are classified into several voltage levels, each having a positive or negative voltage value with a base voltage V.sub.M in the middle, as shown in FIG. 9. A positive voltage and a negative voltage are alternately inverted to each other, for example, every horizontal scanning period.
The data line receiving the output O.sub.n from the driving circuit has an equivalent circuit as shown in FIG. 10. It is necessary for the driving circuit to charge or discharge a capacitance C through a resistance R of the data line in order to apply one of the four gradation voltages V.sub.0 to V.sub.3 to the data line. In FIG. 10, resistance components and capacitance components, which are inherently present in a data line as distributed constants, are equivalently indicated as the resistance R and the capacitance C as lumped elements. Although the data line is further connected to a pixel capacitance C.sub.LC via a TFT as shown in FIG. 10, the pixel capacitance C.sub.LC can be ignored because it has a smaller capacitance by equal to or more than three orders of magnitude than the capacitance C.
When the driving circuit supplies, for example, a gradation voltage V.sub.0 to the data line, as shown in FIG. 11, a voltage of +V.sub.0 is applied to the data line in a scanning period T.sub.1 to charge the capacitance C, and a voltage of -V.sub.0 is applied to the data line in a scanning period T.sub.2 to discharge the capacitance C. In this manner, charge and discharge of the capacitance C are alternately repeated in each horizontal scanning period. When the gradation voltage is switched from V.sub.0 to V.sub.3, a voltage of +V.sub.0 is applied to the data line in the period T.sub.1, and then a voltage of -V.sub.3 is applied to the data line in the period T.sub.2 as shown in FIG. 12. When the gradation voltage is switched from V.sub.3 to V.sub.0, a voltage of +V.sub.3 is applied to the data line in the period T.sub.1, and then a voltage of -V.sub.0 is applied to the data line in the period T.sub.2 as shown in FIG. 13. In this manner, it is necessary to charge or discharge the capacitance C in accordance with the data supplied to the driving circuit.
In such a driving circuit, the difference between the highest voltage +V.sub.0 of the positive gradation voltages and the lowest voltage -V.sub.0 of the negative gradation voltages is taken as, for example, 10 V, and the resistance of one data line is taken as, for example, 50 k.OMEGA.. Under such conditions, the driving circuit of FIG. 8 supplies a charging/discharging current of 0.2 mA (10 V/50 k.OMEGA.) at most. For example, in an RGB display panel having 640 pixels in the horizontal direction, however, the number of the data lines are actually 1920 (640.times.3), and therefore, a driving circuit in such a display panel supplies a maximum charging/discharging current of 384 mA (0.2 mA.times.1920) as a whole.
Therefore, in a conventional driving circuit, it is necessary for the power supply circuits for the gradation voltages V.sub.0 to V.sub.3 to supply a large charging/discharging current by using, as output means of a negative feedback circuit of an operational amplifier 11, a SEPP (single ended push-pull) circuit comprising a complementary symmetry npn transistor 12 and pnp transistor 13, as shown in FIG. 14. Moreover, the analog switches 4 should be bidirectional.
Because of the above, the structures of the power supply circuits and the like are complicated in the conventional driving circuit. As a result, the production cost is raised and a larger electric power is required.